A thin film transistor (TFT) formed over a flat plate such as a glass substrate, which is typically used in a liquid crystal display device, is generally formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. While a TFT using amorphous silicon has low field effect mobility, the TFT can be relatively easily formed over a large-area glass substrate. On the other hand, while a TFT using polycrystalline silicon has high field effect mobility, the TFT requires a crystallization step such as laser annealing and cannot necessarily be easily formed over a large-area glass substrate.
Thus, a technique in which a TFT is formed using an oxide semiconductor as a semiconductor material and applied to an electronic device or an optical device has attracted attention. For example, Patent Documents 1 and 2 each disclose a technique in which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and used for a switching element or the like in an image display device.
A TFT using an oxide semiconductor for a channel formation region (also referred to as a channel region) can have a higher field effect mobility than a TFT using amorphous silicon. An oxide semiconductor film can be formed at a temperature of 300° C. or lower by a sputtering method or the like, and a manufacturing process of the TFT using an oxide semiconductor is simpler than that of the TFT using polycrystalline silicon.
TFTs which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper.
In the case where the TFTs using an oxide semiconductor are applied to a display device, the TFTs can be applied to, for example, TFTs included in a pixel portion or TFTs included in a driver circuit. Note that a driver circuit of a display device includes, for example, a shift register circuit, a buffer circuit, or the like, and the shift register circuit and the buffer circuit further include a logic circuit. By applying the TFT using an oxide semiconductor to a logic circuit of a driver circuit, the driver circuit can be driven at higher speed than in the case of applying a TFT using amorphous silicon.
In addition, the logic circuit can be formed with TFTs all having the same conductivity type. By manufacturing a logic circuit using TFTs all having the same conductivity type, a process can be simplified.